function A0_coef = RK3(A0_coef,dt,N,M,D,B,C)
New_A0 = one_step(A0_coef,N,M,D,B,C);
A1_coef = A0_coef + dt*New_A0;
A1_coef = 0.75*A0_coef + 0.25*A1_coef + 0.25*dt*one_step(A1_coef,N,M,D,B,C);
A0_coef = (1/3)*A0_coef + (2/3)*A1_coef + (2/3)*dt*one_step(A1_coef,N,M,D,B,C);
end